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Skin Pigmentation Generalization theory

Human Skin Colors

Human skin color is determined by a complex set of factors. The most obvious is melanin in the skin, of which there are two types: Eumelanin (brownish pigments) and Pheomelanin (redish pigments).

If we use the LAB color model, we can easily convert a color into theoretical melanin counts. This is overly simplified, but will work fine for our use case.

LAB color model is based off the biology of the human eye. L is how much the Rods are activated. A is how much the red/green cones are activated. B is how much the blue/green cones are activated.

L (lightness) can generally represent how tan the skin is from sun exposure.

Pheomelanin=(B-A)/2

Eumelanin= A + Pheomelanin

Tau Skin Pigmentation assumptions

If we assume that Tau epidermis is also colored via 2 main melanin types, we can assume their equivalent of Pheomelanin is blueish pigments and their equivalent of Eumelanin is greenish pigments. These can be converted in the LAB color space simply as well.

Pheomelanin= -1 * A

Eumelanin= -1 * B

Putting all this together gives us a way to convert human skin tones into their Tau equivalent skin tone.

Update: I swapped Pheomelanin and Eumelanin to get colors closer to the commonly accepted blue tones.

Human tones to Tau tones conversion

This is just a sample and not exhaustive.
#70361c
#1B4A6E
#714937
#3E546C
#65371e
#204562
#492816
#1A3247
#321b0f
#152331
#bf9169
#73A1BE
#8c644d
#567087
#593123
#293D55
#C6775C
#6A8EBC
#8d5524
#1A6A91
#c68642
#379DCB
#e0ac69
#6BC0E6
#f1c27D
#81D4F6
#ffDBAC
#B4E7FF
#ffcd94
#9FE0FF
#ffe0bd
#C7EBFF
#eac086
#8CD0ED
#ffe39f
#A3F1FF
#ffad60
#5EC9FF
#f2efee
#F0F1F2
#efe6dd
#E2E9EF
#ebd3c5
#CBD9E6
#d7b6a5
#ADBDCF
#9f7967
#6F8399
Specify Human Color for conversion
#6A8EBC
Flushed skin (aka blushing) can be calculated by increasing the A value and decreasing the B value to simulate red blood. (This uses the hex color you've chosen).
Use Blue Blood
Human #C6775C#ffDBAC
Tau #6A8EBC #A777CE
Specify Tau Color for conversion
#C6775C

Various tools

Human Skin Tones

Eumelanin range: 17-32

Pheomelanin range: 9-16

Lightness range: 13-92

As you can, see human skin tones go beyond these ranges, but I find in the extremes, you start generating odd color combinations. This is a limitation of the simplified formula.
"#ffffff"
{"eum":0,"phe":0,"l":0}
HEX:#A48F54
#f2efee - phe:0 eum:1 L:95#efe6dd - phe:1.5 eum:3.5 L:92#ebd3c5 - phe:2 eum:8 L:86#d7b6a5 - phe:2 eum:11 L:76#9f7967 - phe:2 eum:14 L:54L:[54,95] a:[1,12] b:[1,16]phe:[0,2] eum:[1,14]
#70361c - phe:2.5 eum:25.5 L:30#714937 - phe:1.5 eum:16.5 L:35#65371e - phe:3 eum:21 L:28#492816 - phe:2.5 eum:15.5 L:20#321b0f - phe:1 eum:11 L:13L:[13,35] a:[10,23] b:[12,28]phe:[1,3] eum:[11,25.5]
#ffcd94 - phe:12.5 eum:22.5 L:86#ffe0bd - phe:7.5 eum:13.5 L:91#eac086 - phe:14 eum:21 L:80#ffe39f - phe:18.5 eum:18.5 L:91#ffad60 - phe:14 eum:37 L:77L:[77,91] a:[0,23] b:[21,51]phe:[7.5,18.5] eum:[13.5,37]
#bf9169 - phe:8 eum:20 L:64#8c644d - phe:3 eum:16 L:46#593123 - phe:0 eum:17 L:25L:[25,64] a:[12,17] b:[17,28]phe:[0,8] eum:[16,20]
#8d5524 - phe:9 eum:28 L:42#c68642 - phe:14 eum:32 L:61#e0ac69 - phe:15.5 eum:26.5 L:74#f1c27D - phe:16.5 eum:24.5 L:81#ffDBAC - phe:11 eum:17 L:89L:[42,89] a:[6,19] b:[28,46]phe:[9,16.5] eum:[17,32]

Tau Skin Tones

"#ffffff"
{"eum":0,"phe":0,"l":0}
HEX:#539AAE
#F0F1F2 - phe:0 eum:1 L:95#E2E9EF - phe:1 eum:4 L:92#CBD9E6 - phe:2 eum:8 L:86#ADBDCF - phe:2 eum:11 L:76#6F8399 - phe:2 eum:14 L:54L:[54,95] a:[-2,0] b:[-14,-1]phe:[0,2] eum:[1,14]
#1B4A6E - phe:3 eum:25 L:30#3E546C - phe:2 eum:16 L:35#204562 - phe:3 eum:21 L:28#1A3247 - phe:2 eum:16 L:20#152331 - phe:1 eum:11 L:13L:[13,35] a:[-3,-1] b:[-25,-11]phe:[1,3] eum:[11,25]
#9FE0FF - phe:13 eum:22 L:86#C7EBFF - phe:7 eum:14 L:91#8CD0ED - phe:14 eum:21 L:80#A3F1FF - phe:20 eum:14 L:91#5EC9FF - phe:15 eum:36 L:77L:[77,91] a:[-20,-7] b:[-36,-14]phe:[7,20] eum:[14,36]
#73A1BE - phe:8 eum:20 L:64#567087 - phe:3 eum:16 L:46#293D55 - phe:0 eum:17 L:25L:[25,64] a:[-8,0] b:[-20,-16]phe:[0,8] eum:[16,20]
#1A6A91 - phe:9 eum:28 L:42#379DCB - phe:14 eum:32 L:61#6BC0E6 - phe:15 eum:27 L:74#81D4F6 - phe:16 eum:25 L:81#B4E7FF - phe:11 eum:17 L:89L:[42,89] a:[-16,-9] b:[-32,-17]phe:[9,16] eum:[17,32]